1. Field of the Invention
This invention relates to a digital servo circuit, and more particularly to such a circuit for digitally controlling rotation of a rotating member with high stability over a wide range.
2. Description of the Prior Art
A servo circuit for an electric motor used in a video tape recorder (VTR) or in similar apparatus must be highly precise, because such apparatus needs high time base stability for a video signal to be properly processed. For reasons of accuracy, cost and stability it is desirable that the servo circuit be formed as a digital circuit.
When adapting a servo circuit for a VTR or like apparatus to digital form, it is convenient for the circuit to be basically similar to a conventional servo circuit of analog form.
FIG. 1 of the accompanying drawings shows one example of a conventional digital servo circuit, which includes a digital rotational speed detector 10 and a digital phase detector 12 which are provided with respective associated digital-to-analog (D/A) converters 11 and 13. A frequency generator (FG) 16 generates a signal representing the rotational frequency of an electric motor 14 which is to be controlled by the servo circuit. The signal from the FG 16 is supplied to the velocity detector 10, and a digital velocity signal developed thereby is converted in the D/A converter 11 into an analog signal x, which is supplied to a circuit point 18.
In addition, a pulse generator (PG) 20 develops a digital signal representing the rotational position or phase of the motor 14. The signal from the PG 20 is compared in the digital phase detector 12 with a reference pulse signal supplied by way of a terminal 22. An error signal produced by the phase detector 12 is converted in the D/A converter 13 into an analog signal y, from which is subtracted the signal x at the circuit point 18. The difference signal resulting at the circuit point 18 is supplied to the motor 14 through a motor drive circuit 24 to control rotation of the motor 14.
The digital servo circuit of FIG. 1, in which the necessary subtraction of signals is performed in the form of analog voltages or current signals, has the advantage that this subtraction is easy to perform, and moreover the necessary circuit constants of the servo circuit can easily be determined. On the other hand, the D/A converters 11 and 13 associated with the digital elements, and in particular the D/A converter 13 associated with the phase detector 12 must have a wide detecting range or dynamic range in order to obtain a satisfactory response speed of the servo circuit.
When the maximum operational range of the D/A converter 13 is wider than that of the D/A converter 11, there is the possibility of the servo circuit behaving abnormally in transient conditions. For example, an excessive error signal can cause the system to go into transient oscillation, or run into a condition beyond the range of lock-in. Therefore, the D/A converter 11 in the servo circuit of FIG. 1 needs a wide dynamic range or resolution for stable operation.
FIG. 2 shows another conventional digital servo circuit, which has a digital adder 26, in which the digital output signals from the detectors 10 and 12 are added together. In the servo circuit of FIG. 2, the adder 26 is provided instead of the D/A converters 11 and 13 associated respectively with the detectors 10 and 12 of the servo circuit of FIG. 1. In this case the digital output of the velocity detector 10 is subtracted from the digital output of the phase detector 12. The output of the adder 26 is converted by a D/A converter 28 into an analog signal, which is supplied to the motor 14 through the motor drive circuit 24.
The servo circuit of FIG. 2 has a similar problem to that of the servo circuit of FIG. 1, that is, in order to prevent the system from behaving abnormally in transient conditions it is necessary for the system to satisfy the condition that m is greater than or equal to n, where the numbers of bits of the outputs of the velocity detector 10 and the phase detector 12 are respectively represented by m and n. Accordingly, when a higher transient lock-in speed is required, it is necessary not only to increase the number of bits n, but also to increase the number of bits m. This adds complication, because of the resultant need to scale up the processing ability of the velocity detector 10 and the adder 26.